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Morphodynamics of two anthropogenically altered tidal inlets

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Title:
Morphodynamics of two anthropogenically altered tidal inlets New Pass and Big Sarasota Pass, west-central Florida
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Language:
English
Creator:
Beck, Tanya M
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University of South Florida
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Subjects

Subjects / Keywords:
Inlet morphology
Anthropogenic activity
Sediment bypassing
Barrier-inlet system
Mixed energy inlet
Dissertations, Academic -- Geology -- Masters -- USF   ( lcsh )
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non-fiction   ( marcgt )

Notes

Abstract:
ABSTRACT: Time-series aerial photographs from 1943 to 2006, including three bathymetry surveys from 1888, 1953, and 2006, are analyzed and compared. The locations of three morphological features, including that of shoreline, offshore bars, and channel orientation, are delineated over the historical aerial photos in order to examine the morphodynamics of the system. Anthropogenic alteration of the New Pass and Big Sarasota Pass system is a crucial factor in controlling the morphodynamics. Both New Pass and Big Sarasota Pass are mixed-energy tidal inlets with New Pass illustrating a straight morphology and Big Sarasota Pass a highly offset morphology. The sediment bypassing at New Pass can be explained by a modified ebb tidal delta breaching model with the breaching initiated by frequent channel dredging. The sediment bypassing at Big Sarasota Pass is different from that at New Pass, in that it is transported across the entire shallow ebb tidal delta with minor interruptions. This particular morphology, without a deep channel in the distal part of the ebb tidal delta, has been maintained by natural processes over at least the last 65 years. The shoreline in the vicinity of both inlets fluctuates as much as 200 m in a time scale of only few years. The advance and retreat of the shoreline at the southern tip of Lido Key is influenced by the frequent Lido Key beach nourishment. A large portion of the sediment is eventually transported onto the Big Sarasota Pass ebb tidal delta. The northern Siesta Key headland has experienced erosion since the 1960s. Downdrift of the headland, a persistent shoreline accretion was observed over the last 40 years, the pattern of which is related to the location and timing of the swash bar attachment.
Thesis:
Thesis (M.S.)--University of South Florida, 2008.
Bibliography:
Includes bibliographical references.
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by Tanya M. Beck.
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Title from PDF of title page.
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Document formatted into pages; contains 137 pages.

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oclc - 319422581
usfldc doi - E14-SFE0002538
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Morphodynamics of Two Anthropogenically Altered Tidal Inlets: New Pass and Big Sarasota Pass, West-Central Florida By Tanya M. Beck A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Geology College of Arts and Science University of South Florida Major Professor: Ping Wang, Ph.D. Richard A. Davis, Ph.D. Mark Luther, Ph.D. Date of Approval: June 25, 2008 Keywords: inlet morphology, anthropogenic activity, sediment bypassing, barrierinlet system, mixed energy inlet, coastal morphology. Copyright 2008, Tanya M. Beck

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Table of Contents List of Figures ii List of Tables v Abstract viI Introduction 1 Objectives 8 Study Area 10 Meteorological Conditions 12 Wave Climate 14 Tidal Regime 16 Sediment Characteristics 19 Trend and Rates of Longshore Sediment Transport 23 Anthropogenic Activities at New Pass and Big Sarasota Pass 24 Methodology 30 Morphodynamics of New Pass and Big Sarasota Pass 34 Location and Orientation of the Inlet Channel 35 New Pass 35 Big Sarasota Pass 41 Shoreline Change 45 New Pass 45 Big Sarasota Pass 59 Offshore Bar 75 Ebb Delta Morphology and Wave Refraction 78 Discussion: A Conceptual Model of the Morphodynamics at New Pass and Big Sarasota Pass 85 Conclusions 92 References 94 Appendix I Rectified Time-ser ies Aerial Photographs 99 i

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List of Figures Figure 1 Davis and Hayes (1984) morphody namic classification of barrierinlet systems including the limit of barrier island formation. 2 Figure 2 Classification of tidal inle ts along the west-central coast of Florida (from Davis and Gibeaut, 1990). 3 Figure 3 Models of inlet sediment bypa ssing (Fitzgerald et al., 1978). 5 Figure 4 Large-scale aggregate sediment bypassing model of inlet system (Kraus, 2000). 6 Figure 5 General study area map. 9 Figure 6 Regional map showing ma jor estuaries including Tampa Bay and Charlotte Harbor. 11 Figure 7 Wind rose diagram showing percentage distribution of measured wind speeds in 2007 at the NOAA Venice Inlet Station. 13 Figure 8 Wave rose diagram of WIS wave hindercast data from 1994-1999 (station 274). 15 Figure 9 Location of the side-looking ADCPs. 17 Figure 10 Hydrodynamic measurem ents at Big Sarasota Pass and New Pass. 18 Figure 11 Tidal phase difference at New Pass and Big Sarasota Pass. 18 Figure 12 Surface sediment gr ain size in phi scale. 21 Figure 13 Surface sediment carbonate percentage. 22 Figure 14 Map of important geographical features. 25 Figure 15 Example of reference c ontrol points selected for aerial photo and navigation map georectified images. 31 ii

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Figure 16 New Pass channel locati on and orientation from 1888 to 2005 illustrated over the 2005 aerial photos. 36 Figure 17 New Pass c hannel aerial photos from 1888 to 1998. 37 Figure 18 Channel orientation of New Pass in 1960. 38 Figure 19 Track of Hurricane Donna, 1960. 39 Figure 20 Aerial photos from 1971 and 1972 illustrating the straight morphology and prominent channel linear bar on the north side of the main channel. 40 Figure 21 Example of delineated updrift edge of inlet channel at Big Sarasota Pass in 1943. 42 Figure 22 Big Sarasota Pass channel location and orie ntation from 1888 to 2005 illustrated over the 2005 aerial photos. 43 Figure 23 Big Sarasota Pass aerial photos from 1888 to 2005. 44 Figure 24 Shoreline change at New Pass from 1943 to 1948. 47 Figure 25 Shoreline change at New Pass from 1948 to 1957. 48 Figure 26 Shoreline change at New Pass from 1957 to 1969. 49 Figure 27 Shoreline change at New Pass from 1969 to 1972. 52 Figure 28 Shoreline change at New Pass from 1972 to 1977. 53 Figure 29 Shoreline change at New Pass from 1977 to 1986. 55 Figure 30 Shoreline change at New Pass from 1986 to 1998. 57 Figure 31 Shoreline change at New Pass from 1998 to 2006. 58 Figure 32 Shoreline change updrift of Big Sarasota Pass from 1943 to 1948. 60 Figure 33 Shoreline c hange downdrift of Big Sa rasota Pass from 1943 to 1948. 62 Figure 34 Shoreline change updrift ofBig Sarasota Pass from 1948 to 1969. 63 Figure 35 Rapid development of Bird Key from 1959 to 1960. 64 iii

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Figure 36 Shoreline c hange downdrift of Big Sa rasota Pass from 1948 to 1969. 66 Figure 37 Shoreline change updrift of Big Sarasota Pass from 1969 to 1983. 67 Figure 38 Shoreline c hange downdrift of Big Sa rasota Pass from 1969 to 1983. 69 Figure 39 Shoreline change updrift of Big Sarasota Pass from 1983 to 1999. 70 Figure 40 Shoreline c hange downdrift of Big Sa rasota Pass from 1983 to 1999. 71 Figure 41 Shoreline change updrift of Big Sarasota Pass from 1999 to 2006. 73 Figure 42 Shoreline c hange downdrift of Big Sa rasota Pass from 1999 to 2006. 74 Figure 43 Example of offshore bar detachment point and attachment point. 75 Figure 44 Offshore bar locations fo r south Longboat Key from visible aerial photos of New Pass 76 Figure 45 Offshore bar locations fo r Lido Key from visible aerial photos of New Pass and Big Sarasota Pass. 78 Figure 46 Bathymetric map of study area used in modeling. 80 Figure 47 Wave propagation of a 1. 0 m wave height with a 5 s period approaching from a north-northwest direction. 82 Figure 48 Wave propagation of a 2. 4 m wave height with an 8 s period approaching from due west. 83 Figure 49 Wave propagation of a 1. 2 m wave height with a 6 s period approaching in from the south. 84 Figure 50 Illustration of pathways of natural sediment bypassing at New Pass. 88 Figure 51 Illustration of pathways of natural sediment bypassing at Big Sarasota Pass. 89 iv

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List of Tables Table 1. Historical inlet dredging and nou rishment projects on both Longboat Key and Lido Key. 28 v

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Morphodynamics of Two Anthropogenically Altered Tidal Inlets: New Pass and Big Sarasota Pass, West-central Florida Tanya M. Beck Abstract Time-series aerial photographs from 1943 to 2006, including three bathymetry surveys from 1888, 1953, and 2006, are analyzed and compared. The locations of three morphological f eatures, including that of shoreline, offshore bars, and channel orientation, ar e delineated over the historical aerial photos in order to examine the morphody namics of the system Anthropogenic alteration of the New Pass and Big Sarasota Pass system is a crucial factor in controlling the morphodynamics. Both New Pass and Big Sarasota Pass ar e mixed-energy tidal inlets with New Pass illustrating a straight mor phology and Big Sarasota Pass a highly offset morphology. The sediment bypa ssing at New Pass can be explained by a modified ebb tidal delta breaching model with the breaching in itiated by frequent channel dredging. The sediment bypassing at Big Sarasota Pass is different from that at New Pass, in that it is transported across the entire shallow ebb tidal delta with minor interruptions. This particular morphology, without a deep channel in the distal part of the ebb ti dal delta, has been maintained by natural processes over at least the last 65 years. The shoreline in the vicinity of both vi

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vii inlets fluctuates as much as 200 m in a time scale of only few years. The advance and retreat of the shoreline at t he southern tip of Lido Key is influenced by the frequent Lido Key beach nourishment. A large portion of the sediment is eventually transported onto the Big Saraso ta Pass ebb tidal delta. The northern Siesta Key headland has experienced erosi on since the 1960s. Downdrift of the headland, a persistent shoreline accretion was observed over the last 40 years, the pattern of which is related to the location and timing of the swash bar attachment.

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Introduction Tidal inlets play an important role in nearshore processes along barrier island coastlines. Escoffier (1940, 1977) describes an inlet as a short, narrow waterway which connects a bay, lagoon, or estuary to a lar ger body of water, facilitating exchange of water, sediments, nutrients, and pollutants. The presence of an inlet along the coastline traps a considerable amount of sand, thereby creating the potential for er osion of the adjacent beaches (Dean and Dalrymple, 2002). For example, Dean ( 1988) suggested that 80% of the east coast of Floridas shoreline erosion can be directly linked to tidal inlets. Understanding the processes of tidal in lets and their influence on morphologic change provides crucial insight into regional behavior of barrier island coastlines. Coastal inlets, particularly those in Flor ida, tend to be heavily modified by anthropogenic activities. Therefore, a comprehensive understanding of inlet morphodynamics is also essential for coastal management. A tidal inlet presents a break in an otherwise continuous barrier island, interrupting the pathway of longshore sedi ment transport driven by obliquely incident waves. The longshore moving sa nd may be redistributed both landward and seaward by flooding and ebbing tidal currents, forming flood and ebb tidal deltas. In other words, ti dal inlets effectively act as sediment traps for longshore moving sand. Therefore, the balance between longshore sediment transport and 1

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the strength of tidal flow dictates the mo rphological characteristics of tidal inlets (Bruun, 1960; Hayes, 1979). The morphodynamics of many coastal systems are often characterized in terms of relative dominance of wave or tidal forcing (Hayes, 1975; 1979). Davis and Hayes (1984) developed a morphodynamic classification of coastal systems, emphasizing barrier-islands (Figure 1). T he low-energy Florida Gulf of Mexico coast is located near the origin of Figure 1. Therefore, a sm all change in either tidal range or wave height will cause a s ubstantial change in morphology (Davis and Barnard, 2003). This delicate balance between the relative forcing of tides and waves results in all varieties of c oastal morphodynamics ranging from tidedominated to wave-dominated systems along the Florida Gulf coast. Figure 1. Davis and Hayes (1 984) morphodynamic classifi cation of barrier-inlet systems including the limit of barrier island formation. 2

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Davis and Gibeaut (1990) applied the Davis and Hayes (1984) classification to tidal inlet morphodynam ics along the west-central Florida coast (Figure 2). They identified four types of tidal inlet systems including tidedominated, mixed energy straight, mix ed energy offset, and wave-dominated tidal inlets. Tide-dominated inlets typi cally have a deep and stable channel with extensive ebb and flood tidal deltas. Wave-dominated inle ts are characterized by unstable and migratory channel s with a typically small a nd asymmetric ebb tidal delta. In some cases the ebb tidal delta may even be absent. Under mixed energy settings, the morphological charac teristics associated with both wave and tide forcing are apparent. Dependent upon t he particular pattern of sediment bypassing, mixed energy inlets may exhibit either a strai ght or offset morphology. Figure 2. Classification of tidal inlets along the west-central coast of Florida (from Davis and Gibeaut, 1990). 3

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Comprehensive understanding of sediment bypassing at tidal inlets is essential in inlet and barrier-island morphodynamics. Fitzgerald (1988) developed three models for mechanisms and patterns of sediment bypassing (Figure 3). The three majo r mechanisms for inlet sedim ent bypassing include: 1) inlet migration and spit breaching, 2) l andward migration of bar complexes at stable inlets, and 3) ebb tidal delta breaching. The trend of bypassing under models 1 and 3 is rather apparent and tend to be event related. As the inlet channel is further and further skewed dow ndrift driven by longshore sediment transport, the decreasing inlet hydraulic e fficiency may lead to breaching. As the newly breached inlet establishes itself, t he part of barrier island (model 1) or ebb tidal delta (model 3) that was at the updr ift side of the inlet, becomes effectively bypassed to the downdrift side of the new inlet. The trend of bypassing under model 2 with a stable channel is not as obvious as models 1 and 3. Complex and case specific movement of c hannel margin linear bars and swash bars constitute the sediment pathways. 4

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Figure 3_. Models of inlet sediment bypassing (from Fitzgeral d et al., 1978). 1) inlet migration and spit breaching; 2) stable inlet processes; and 3) breaching of ebb tidal delta by re location of main ebb-channel. In contrast to numerous studies on inlet hydrodynamics and stability (Bruun, 1978; Metha and Ozsoy, 1978; V an de Kreeke, 1972; 1988; Aubrey and Giese, 1993), detailed mathematical modeling of long-term and large-scale morphology change at inlets is a relative ly new area of research (De Vriend, 1996a, and b). Resolving fi ne-scale processes of s ediment transport in the vicinity of a tidal inlet is very difficul t. In addition, large-scale morphology change requires computation at a much larger temporal scale than those used in hydrodynamic and sediment transport co mputations. As an alternative, aggregate modeling of large-scale geomorphic features based on a small number of attributes has been attempt ed (Stive et al., 1998; Kraus, 2000). 5

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Kraus (2000) developed a reservoir model of ebb tidal delta evolution and sand bypassing. A large-scale aggregate model based on fundamental attributes can utilize a much larger spatial resoluti on as well as longer time scales that are associated with the entire mor phological form of an ebb s hoal (Kraus, 2000). An integral assumption included in the model is that the longshore transport defines the type and amount of sediment of which the ebb shoal is composed. A general set of initial assumptions for aggregate models included in Kr aus (2000) are 1) Mass is conserved, 2) Morphological forms and the sediment pathways are identifiable throughout ev olution of the feat ure (Figure 4A), 3) Stable equilibrium of the individual morphologi c form(s) exist (Figure 4B), and 4) Changes in mesoand macro-morphological forms are reasonably smooth. Figure 4. Large-scale aggregate sediment bypassing model of inlet system (Kraus, 2000). A is a conceptual model and B is the sediment budget of the aggregate model. Recent improvements in surveying, remote sensing, and data analyses technology allow for better quantificat ion of both inlet processes and the resultant morphology for development of predictive relationships and further improvement upon numerical models (Fitzgerald et al., 2003). Fitzgerald et al. (2003) discussed the applications of updat ed measurement technology, including Light Detection and Ranging (LIDAR), si de-scan sonar, acoustic Doppler current 6

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profilers (ADCP), and Ground Penetration R adar (GPR) in comprehensive inlet studies. In addition, Fitz gerald et al. (2003) also emphasized the application of Geographical Information System (GIS) tools in compilati on and analysis of large datasets. Detailed t opographic, bathymetric, hydrodynamic, and other geophysical data collected us ing the aforementioned technology is typically limited to a short time fram e. In contrast, a large amount of historical data on tidal inlets, especially in Florida, is av ailable through time-series aerial photos. These aerial photos can be accurately and efficiently rectified using GIS technology. This allows a semi-quantitative analysis of morphological changes over an extensive time scale. The West-central Florida coastline has 29 barrier islands, 30 tidal inlets, and the most diverse morphology of any barrier system in the world (Davis, 1989). A large range of tidal inlets, in terms of their morphodynamics, is found along this coast. Davis and Gibeaut (1990) and Gibeaut and Davis (1993) summarized the morphological characterist ics of ebb tidal deltas along this coast. Dean and OBrien (1987) exam ined the interaction between tidal inlets and the adjacent shoreline along the Florida west coast. Davis and Barnard (2000) analyzed the influence on t he anthropogenic modifications in the back-barrier area on tidal inlet stability. Wilhoit (2004) investigated the morphodynamics of Bunces Pass, a pristine tide-dominated inlet situated near the mouth of Tampa Bay. Wang et al. (2007) examined the hydrodynamics and morphodynamics of a heavily structured and wave-dominated inlet at Blind Pass in Pinellas County. 7

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Mehta et al. (1976) examined various factors controlling the hydrodynamics and sediment transport processes at both Johns Pass and Blind Pass. New Pass and Big Sarasota Pass are situated along the microtidal, lowwave energy coast of West-central Florid a (Figure 5). The two closely spaced inlets carry a relatively large tidal prism, on the order of 107 m3 each, draining a large portion of Sarasota Bay. Based on the classification (Figure 2) of Davis and Gibeaut (1990), New Pass inlet has a mi xed-energy strai ght morphology and Big Sarasota Pass has a mixed-energy o ffset morphology. Both inlets have relatively stable main channels and la rge ebb tidal deltas. Several inlet management studies for each inlet were c onducted to investigate inlet stability and potential sand resources of ebb tidal deltas (CPE, 1993). Objectives This study focuses on analysis and comparison of time-series aerial photographs of these two inlets from 1943 to 2006. Three bathymetry surveys from 1888, 1953, and 2006 are al so compiled for analysis and comparison. All the data are digitized and compiled using Arc GIS 9.2. Digital aerial photos are of high geometric capability and are a useful s ource of data because the study area has low topographic relief (Fitzgerald et al., 2003). CMS-Wave (Lin et al., 2006) is used to examine wave propagation pattern s over the ebb tidal delta complex. The objectives of this paper are to examin e 1) the various factors controlling the morphodynamics of the two inlets, 2) in teraction between the inlets and the 8

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adjacent beaches, and 3) morphodynamic response of the inlets and the adjacent beach to anthropogenic modifications. Figure 5. General study area map. 9

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Study Area Located along the western Florida Gu lf of Mexico coast, New Pass and Big Sarasota Pass serve the southern por tion of Sarasota Bay, immediately south of the Tampa Bay Estuary (Fi gure 6). The Intercoastal Waterway hydraulically links Sarasota Bay to Tampa Bay to the north and Little Sarasota Bay to the south. New Pass separate s the 16-km long Longboat Key to the north and the 4-km long Lido Key to the south. Big Sarasota Pass separates Lido Key and a long barrier island system to the south. Siesta Ke y, a drumstick barrier, is located at the north end of this barrier system. The general study area coastline has a northwest to southeast orientation. Most of the tidal prism that flows through New Pass and Big Sarasota Pass like ly comes from the southern portion of Sarasota Bay. Longboat Pass, the third inlet serving Sarasota Bay, is located at the northern end of the bay 16 km north of New Pass. This long distance, in addition to a possible restriction caused by Long Bar (Figure _) should limit the interaction between New Pass-Big Saraso ta Pass and Longboat Pass in terms of morphodynamics on a decadal scale. The water body to the south of New Pass and Big Sarasota Pass is rather narrow and restricted. The closest tidal inlet to the south is Venice Inlet, approximately 22 km from Big Sarasota Pass. The interaction between Venice Inlet and the two study inlets is minimal. 10

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Figure 6. Regional map showing ma jor estuaries including Tampa Bay and Charlotte Harbor. 11

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Meteorological Conditions Due to the limited fetch of the Gulf of Mexico, waves are controlled by both regional and local meteorolog ical conditions. Also, bec ause the microtidal nature of the greater study area, the tidal r ange may be significantly influenced by meteorological conditions Therefore, local and r egional meteorological conditions may have substantial influence on the inlet morphology. There are two distinct seasonal weather patterns associated with the low latitude of the study area (Davis and Barnard, 2003). The Bermuda High dominates the regular summer pattern with gentle easterly wi nds. High energy events during the summer are associated with passages of tropical storms, however, a direct hit by a hurricane strength tropica l storm is uncommon. The last such storm that passed within 40 km from the study area was an unnamed hurricane in 1946. During the winter season, the frequent pa ssage of frontal systems is the main source for high wave-energy events. The sustained and relatively strong northerly wind after the front al passage is the major cause of southerly longshore sediment transport (Davis and Barnar d, 2003; Elko et al., 2005). The distribution of wind speed and direction measured during 2007 at Venice Inlet is summariz ed in Figure 7. Throughout much of the year, wind speeds are typically less than 6 m/s, shown in orange in Figure 7. The predominant wind direction is from the nor theast usually with a slow speed. The 12

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influence of cold front passages is apparent as indicated by the secondary mode (shown in light blue color) approaching fr om the northwest, often proceeded by a strong pre-frontal wind from a southerly direction. The pre-frontal wind, although can be quite strong, typically only lasts a short period of time (Tidwell, 2005). This can also be implied from the much narrower light-bl ue bar in Figure 7. It is worth noting that there was no passage of significant tropical storms during 2007. Figure 7. Wind rose diagr am showing percentage dist ribution of measured wind speeds in 2007 at the NOAA Venice Inlet Station. 13

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Wave Climate The overall wave energy along this coast is low with average breaker heights for west-central Florida estima ted to be 25-30 cm (Tanner, 1960; Davis and Andronzco, 1987). Figure 8 is a summary of hindcast wave conditions from 1994 to 1999 based on the Wave Information Study (WIS) by the US Army Corps of Engineers. The data illustrated are from WIS station 274, which is located in 18m of water depth approximately 26 kilometers offshore to the west of the study area. Most of the time the significant wave height is less than 1 meter, approaching from an easterly direction (Figure 8). These offshore directed waves have a minimal effect on the nearsh ore processes of the inlets. The relationship between wave conditions and the passages of cold fronts is apparent with higher waves approaching from the we st-northwest direction (Figure 8). These highly oblique waves have a significant impact on the nearshore processes and the resultant inlet morphology. Wave-induced sediment transport in t he study area is episodic, controlled by the high-energy events associated wit h cold front passages. The wind and waves during these events typically are inci dent from a northerly direction, driving a southward longshore sedim ent transport, as also f ound by numerous previous studies (Davis and Barnard, 2003; Elko et al., 2005). However, during the rest of the year, wave forcing should not be signifi cant. On a smaller temporal scale, the 14

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sea breeze during the summer season may generate modest waves in the nearshore. However, their effect on sediment transport is insignificant. Figure 8. Wave rose di agram of WIS wave hindcas t data from 1994-1999 (station 274). 15

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Tidal Regime Two months of tide and current measurements were conducted simultaneously at New Pass and Big Sarasota Pass by this study using two sidelooking ADCPs. The locations of the measurements are shown in Figure 9. Details of this hydrodynamic study are beyond the scope of this thesis. Data are briefly summarized to provide general char acteristics of the ti dal regime in the following section. Tides in the region are classified as mixed-tropical tides with a microtidal range (Davis and Barnard, 2003). The spring tide is typically diurnal with a range of roughly 0.8 m, while the neap tide is se mi-diurnal with a range of 0.3 to 0.4 m (Figure 10A). Although the diurnal spring tidal range is nearly twice as much as the semi-diurnal neap tidal range, the peak velocities through both channels are largely similar due to a similar rate of wa ter level change. During spring tide the rising phase occurs over a longer period of time than the falling phase, resulting in a much stronger ebb flow at both inle ts (Wang et al., 2007). Ebbing velocities at both inlets typically reach or sur pass 1.5 m/s during spring tide, and flood velocities typically peak at 1.0 m/s (Figure 10B). Both spring and neap flooding tides at Big Sarasota Pass lead New Pa ss by 20 to 60 minutes; however, the falling tide is largely in phase (Figure 11). 16

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Based on estimation by CPE (1993), the tidal prism through New Pass is roughly 1.1 x 107 m3 and the Big Sarasota Pass prism is approximately twice that of New Pass at 2.1 x 107 m3. The tidal prism during spring tide calculated based on the flow measurement from this study yielded a similar tidal prism. Figure 9. Location of t he side-looking ADCPs. 17

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Measured Tide-0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 4/1/20064/4/20064/7/ 20064/10/20064/13/20064/16/20064/19/2006Time (days)Elevation MWL (m) BigPass Tide NewPass Tide Tidal Velocity at Big Sarasota Pass-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2/24/20062/27/20063/2/20063/5/20063/8/20063/11/2006Time (days)Velocity (m/s) Figure 10. Hydrodynamic measurements at Big Sarasota Pass and New Pass. A) Tidal water-level fluctuation m easured at both Big Sarasota Pass and New Pass between 4/1/2006 to 4/20/2006. B) Averaged crosschannel tidal velocity calculated at Big Sarasota Pass between 2/24/2006 to 3/14/2006. Measured Tide-0.6 -0.4 -0.2 0 0.2 0.4 0.6 2/28/2006 3/1/2006 3/2/2006Time (days)Elevation MWL (m) BigPass Tide NewPass Tide Figure 11. Tidal phase difference at New Pass and Big Sarasota Pass. 18

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Sediment Characteristics Sediments along the Gulf coast of Florida are dominated by fine quartz sand with varying amounts of shell debris (Evans et al, 1985). Ther e is little to no riverine input of sediment to the coas tal system, and unconsolidated sediment cover rapidly thins in the offshore direct ion (Brooks et al, 2003a; Twichell et al, 2003; Davis and Kuhn, 1985). At the inlet and adjacent shoreline, sediment is mainly composed of fine to very fine quar tz sand with varying amounts of gravelsized shell and negligible amounts of biogenic mud. Sixty surface sediment samples were collected on the flood tidal delta, in the main channel, and on the ebb tidal delta for this study. Details of the sediment analysis are beyond the scope of this thesis, however, general characteristics of surface sediment samp les are presented for the study area. Figure 12 shows the location and mean sediment grain size (phi) of the sediment samples. Figure 13 shows the carbonate concentrations, which effectively illustrate the fractions of s ediment other than the fine to very fine quartz sand. In the inlet channel, sediment varies greatly in mean grain size from 0.16 mm (2.64 phi) to 10 mm (-3.32 phi), wit h carbonate concentration varying from roughly 2% to 100%. The coarse, shelly sediment s are channel lag deposits consisting of predominantly biogenic shell hash, found mostly in the deepest part of the channel thalweg. The fine quartz sand with minimal shell content is mostly found 19

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along the slope of the channel, where large sand wave s are often observed. A variety of sediment textures are f ound in between the above two end members. Flood tidal delta sediment characterist ics are relatively uniform with a mean sediment grain size rangi ng from 0.13 mm (2.94 phi) to 0.20 mm (2.32 phi) with little to no shell material. The finer sediments have a small content of organic mud, which is the primary s ource of mud sized grains in the greater study area. The mean sediment grain size on the ebb ti dal deltas varies over a greater range than that on the flood tidal delta, varying from 0.15 (2.74 phi) to 0.3 mm (1.74 phi) controlled by the various amount of she ll debris. Field observations indicated that the shell debris tend to distribute in a patchy pattern. 20

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Figure 12. Surface sediment grain size in phi scale. Samples collected in 2006 for the Sarasota Inlet Management Plan. 21

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Figure 13. Surface sediment carbonate percentage. Samples collected in 2006 for the Sarasota Inlet Management Plan. 22

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Trend and Rates of Longshor e Sediment Transport The net and gross rates of longshore s ediment transport play a significant role in inlet stability and morphology. Walton (1976) estimated net and gross longshore sediment transport rates for We st-central Florida based on estimated breaking wave conditions. Most of the s outherly transport occurred in the winter months, influenced by the passage of cold fronts. Waltons (1976) estimate has been used in various inlet management studies (CPE, 1993); however, the uncertainty associated with Waltons calculations is unknown. In addition, Waltons (1976) regional estimation does not resolve local variation in longshore sediment transport, especially in the vicini ty of tidal inlets in this study. Probably, a more accurate way to estimate trends of longshore sediment transport rate, along a complicated barrier-i nlet coast, is through analysis of timeseries morphology change. The southerly net longshore transport is clearly illustrated by the orientation of numerous morphological features. This study focuses on resolving morphology change through analysis and comparison of rectified time-series aerial photos. Tr ends and patterns of longshore sediment transport can be inferred from the morphology analysis. 23

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Anthropogenic Activities at New Pass and Big Sarasota Pass A hurricane in 1848 opened New Pass creating a wide gap separating Longboat Key with Siesta Key (Harvey, 1982). The age and origin of Big Sarasota Pass are unknown although the age of adjacent Siesta Key has been documented at about 3000 BP by Stapor et al (1988). This makes Siesta Key one of the oldest barrier islands on t he west central Florida coast. The prograding beach/dune ridges on Siesta Key are at least 2000 years old, implying that an inlet has existed at that location since that time. Anthropogenic activities have played a significant role in the evolution of both New Pass and Big Sarasota Pass over the last century. As a matter of fact, the very existence of the clearly defined New Pass and Big Sarasota Pass is attributed directly to the artificial creation of Lido Ke y. World War II marks a period of extensive settlement along this coast as well as a change in the approach and intensity of anthropogenic mo difications to the natural system. Historical records extend back to the first Europeans traveling along the coastline noting features such as the locations of major inlets, including the observation of Boca Sarasota as an entranc e into Sarasota Bay (Figure 14). The oldest maps in this area of the coast date back to the early 1800s, and included Fishery Point, a fishing town located on the north side Sarasota Key (now known as Siesta Key) along the channel of Big Sarasota Pass. 24

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Figure 14. Map of impo rtant geographical features. 25

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Since the opening of New Pass in 1848, both New Pass and Big Sarasota Pass carried a significant portion of the ti dal prism of southern Sarasota Bay. The inlets were separated by grass flat s and a group of small mangrove islands known as the Cerol Isles (CPE, 1993b). A historical map of Sarasota dating back to 1888 illustrates that both the size and orientation of New Pass and Big Sarasota Pass are generally similar to the present. Much of the initial dredge and fill ac tivities in Sarasota Bay were conducted along the back-bay area. The fi rst bridge, now known as the Siesta Key Bridge, was built in 1917 connecting the mainland to Sarasota Key (Figure 14). The Hanson, Nettie and Louise Bay ous were dredged to create the first platted subdivision with canals in Saraso ta named Siesta. Sarasota Key later becomes known as Siesta Key. In 1912, John and Charles Ringling purchased much of the land surrounding both inlets and along the ba yside including the Cerol Isles. Beginning in the late 1910s and earl y 1920s, John Ringling built Ringling Causeway from the mainland to the Cero l Isles, and then further connected them to Longboat Key. Along the Ri ngling Causeway a series of artificially expanded islands were built over originally mangrov e islands or grassfla ts. Seaward from the mainland, the three most prominent artificial islands are Cedar Key, Bird Key, and St. Armands Key (Figure 14). The construction of this causeway and artificial islands may have altered the bay circulation and the tidal prisms of New Pass and Big Sarasota Pass. 26

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The most substantial modification to the natural system was the creation of Lido Key. Lido Key was built by filling in the Cerol Isles with dredged material excavated from Sarasota Bay. Lido, Italian for beach, was chosen as a Mediterranean themed name for the newly cr eated island. In 1926, the city of Sarasota dredged New Pass and placed the material along the north side of Lido Key. The nearly 60 acre extension of north Lido Key, also known as City Island, substantially changed the New Pass channel configuration (Figure 14). It was not until the post-war 1940s and 1950s that the coast really began to develop and grow in population. The last major change to the bay was the creation of Bird Key in 1959 (Figure 14). The new subdivision was more than twenty times the original size from 14 acres to approximately 300 acres. This caused substantial degradation to the bay fisheries and caused an environmental uproar over the development. No further development of bay property has been approved since then. Both the Lido Ke y and south Longboat Key coasts were developed into popular winter resort desti nations and residential villages by the early 1960s. Associated with the increasing devel opment was a series of nonintegrated efforts to stabilize both the bayside and Gulf -side shoreline to protect from wave and current induced erosion. Sea walls were built around much of the bay, as is common in west-central Flor ida. Also, seawalls we re constructed along the southern side of both inlets, along with additional reinforcement with rip-rap and groin fields. This effectively halted the southerly migration of the inlets. Along the Gulf beaches, groin fields were also used as a shoreline protection measure. 27

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In contrast to hard engineering struct ures, that dominated before the mid1960s, dredging and beach nourishment became the primary shore protection measure. In 1964, New Pass was authorized as a F ederal navigation project and was dredged and realigned perpendicular to the overall shoreline trend (USACE, 1968). The dredged material was placed on Lido Key to help alleviate erosion along the central and south beac hes. Since 1964, channel dredging at New Pass and subsequent beach nourishment have become a regular method for navigational channel maintenance and b each erosion control. Table 1 lists the date and sand volume of each dr edge event, and the location of the associated beach nourishment. The central beach of Lido Key has been identified as an erosional hot spot since the 1960s (USACE, 1968). Lido key has been renourished a total of 11 times with eight of the projects using material from the maintenance dredging of New Pass (CPE, 1993b). The first nourishment was in 1964 when 93,000 m3 of sand were dredged from the New Pass channel and placed onto Lido Key (CPE, 1993b). The most recent nourishment of Lido Key occurred in 2003. In 1964, the Army Corps of Engi neers decided against selecting Big Sarasota Pass as a priority navigation in let because of its large and complex ebb tidal delta (CPE, 1993a). D ue to the large offset at Big Sarasota Pass, the ebb tidal delta is perceived by north Siesta Key residents as providing a major sheltering from northerly approaching wave s. Any attempt to dredge the large ebb delta was strongly opposed by the Siesta Key community. As a result, Big 28

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Sarasota Pass and its ebb tidal delta have never been dredged. Also, north Siesta Key has never been nourished. Table 1. Historical inlet dredging and nourishment projects on both Longboat Key and Lido Key. Note most of all New Pass dredged material is placed on the downdrift Lido Key. YearSourceDredged Quantity (m3) Quantity placed on Lido Key (m3) Quantity placed on Longboat Key (m3)1964*New Pass92970 90920 2050 1970*Offshore262960 262960 1974*New Pass184900 184900 1977*New Pass300500 300500 1982*New Pass139120 69120 70000 1985**New Pass142000 142000 1990/91**New Pass270470 180320 90160 1993New Pass601050 390680 210370 1997 New Pass244930 122460 1998 Offshore214120 214120 2001 Offshore270470 270470 2003 New Pass93910 93910 U.S. Army Corps of Engineers (April 1984) ** CPE (1991) Sarasota CO. (2005)Modified from CPE (1992)Historical Nourishment Quantities on Longboat and Lido Keys In summary, the anthropogenic activity at New Pass and Big Sarasota Pass differs significantly in that the New Pass channel is dredge of frequently, while no dredging activities have occurred at Big Sarasota Pass. Most of the dredged material is placed along the Li do Key beaches, directly updrift of Big Sarasota Pass. However, there are similar anthropogenic modifications to both inlets, including construction of sea walls and groin fields along the downdrift side of the channel. Also, neither inlet has je tties, which are fairly common at other developed inlets in Florida. 29

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Methodology The primary goal of the study is to understand the mor phodynamics of the two closely spaced inlets through the analysis of time-series rectified aerial photos. Aerial photos and navigation maps were digitized and rectified using ESRI ArcGIS software. All images were rectified using land-feature control points on each aerial image over a refer enced digital orthophoto. The Digital Orthophoto Quarter Quad (DOQQ) was obtained from the Land Boundary Information Service (LABINS) of the Bu reau of Survey and Mapping, Florida. DOQQ imagery has an image resolution of 1 meter per pixel, and a horizontal accuracy of 0.18 m. All DOQQ images ar e in compliance with the National Map Accuracy Standards (NMAS). Distinguishable fixed features, in cluding road intersections, seawall corners, and building corners were select ed in all aerial photos for control points, and an example is illustrated in Figure 15. At least six control points selected for the rectification. Emphasis was placed on the features located near the inlet. This was done to ensure the highest accuracy of referencing in the vicinity of the features of interest. Also, for most of the cases, a considerable portion of the image is covered by water, and selecting co ntrol points over this featureless area is difficult. Therefore, t he accuracy of the rectificatio n decreases over relatively 30

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wide water bodies, e.g., along the western (Gulf) and eastern (bay) boundaries of many photos. Appendix I includes all the rectified aerial photos. Figure 15. Example of reference cont rol points selected for aerial photo and navigation map georectified images. 31

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After the images are rectified, they c an be easily overlain and compared to examine morphologic change. Furthermore in order to quantify the changes of important morphologic features, including shoreline, updrift edge of the channel, and offshore bars, these features are delinea ted using a digitizing tool in a GIS. Shoreline positions were determi ned manually, based on the color change between land pixels and shallo w water pixels. Consider ing that tidal ranges are typically less than 0.6 m, the influence of tidal fluctuation on the location of the shoreline should not be significant for a study at this scale. In addition, a section of a stable and natural shoreli ne in the backbay was digitiz ed as a control. If the digitized stable shoreline remains at a si milar location for the time-series aerial photos, then this may indicate that the uncertainty in shoreline position associated with the water-level fluctuation is not significant for this analysis. The crests of offshore bars were delineated through visual interpretation of the brightest pixels on an image taken on a clear day, or offshore breaking waves found in an image taken on a day with high er wave action. The updrift edge of the channel, including the channel margin linear bar, was digitized as an indicator of the channel location and orientation. This feature was chosen over the channel thalweg to indicate channel loca tion and orientation because the latter was impossible to identify in the aerial photos. Comparison of the digitized shoreli nes provides information on the trends of beach erosion and accretion. Information on the morphodynamics of the ebb tidal delta can be inferred by the orient ation change of the main channel over the delta. Based on Fitzgerald (1988) and Elko and Wang (2007), the nearshore bar 32

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may provide an important sediment pat hway from the beach to the ebb delta complex. Therefore, the offshore bar is delineated in this study to investigate the relationship of the offshore bar and the trend of sediment by passing to the ebb tidal delta. Also, in order to exami ne the driving mechanism for sediment bypassing over the ebb tidal delta, wave propagation modeling is conducted. Anthropogenic modifications, includi ng bay area changes, beach nourishment, and inlet dredging activities, are linked to the time-series morphology changes. 33

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Morphology of New Pass and Big Sarasota Pass Time-series aerial photos of New Pass and Big Sarasota Pass, from 1943 to 2006, are rectified and analyzed. In addition, three bathymetric datasets are examined, including two from rectified navigational maps in 1888 and 1953 and one from recent a survey in 2006. The focus of this analysis is on sediment bypassing mechanisms and pathways based on investigations of various morphological features in the inlet system. The features investigated include the inlet channel, the offshore bar, shore line along the adjacent beach, and the ebb tidal delta. In the following, time-ser ies evolution of the above features is discussed largely on a decadal scale. Anthropogenic modifications play a significant role in the morphological evolution and are em phasized throughout the discussion. Generally, t here are two types of ant hropogenic modifications. Before the mid-1960s, hard engineering stru ctures, for example sea walls, groin fields, and dredge-and-fill in the bay, are t he dominant activities. Following the late-1960s, the above activities were largely replaced by beach nourishment and inlet maintenance dredging. The only available data that illustrates minimal anthropogenic modifications is t he navigational map from 1888. 34

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Location and Orientation of the Inlet Channel New Pass The location of the New Pass channel, delineated from the 1888 navigational map, is consi derably north of the present location (Figure 16). The channel is curved toward the south, apparent ly influenced by net littoral drift in the southerly direction. This indicate s a greater degree of wa ve influence of the inlet before anthropogenic modifications. T he first dredging of New Pass was in 1926 with the intention of increasing hydraulic st ability (CPE, 1993b). The location of the New Pass ch annel, delineated from the first aerial photo in 1943, is considerably sout h of the 1888 location. The channel has migrated up to 260 meters southward. Furthermore, the 1943 image shows a relatively straight channel (Figure 17), in contrast to the curved channel in 1888. The channel is not as visible in the 1957 aerial photo. However, a relatively straight channel over the ebb tidal delta is illustrated. Over all, during the 1940s and 50s, New Pass had a predominantly mix ed-energy straight morphology with a long and straight channel margin linear bar. 35

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Figure 16. New Pass channel locati on and orientation from 1888 to 2005 illustrated over t he 2005 aerial photos. 36

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Figure 17. New Pass channel ae rial photos from 1888 to 1998. 37

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The overall orientation of the ent ire channel, delineat ed from the 1960 aerial photo (Figure 18), is substantially different from that in the 1940s and 1950s due to the different ebb tidal delta morphology. However, the part of the channel between the barrier islands rema ins at a similar location. The substantial change of the channe l orientation over the ebb tidal delta is the result of two factors. As typical of west-centra l Florida coast, dredge and fill activity in the back-barrier bay was quite active in the 1950s (Davis and Barnard, 2003). For this study area, this dredge and fill ac tivity is largely concluded by the construction of Bird Key in 1959. These activities resulted in a decrease of bay area, and therefore a decreas e in the tidal prism. Figure 18. Channel orientation of New Pass in 1960. 38

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The second reason for the curved 1960 channel is related to the passage of Hurricane Donna. Hurricane Donna stru ck South Florida in early September of 1960, and moved up the state near and landward of the west-central coast (Figure 19). Donna was a category 4 hurricane on the Saffir-Simpson scale that moved very slowly across the state. After the storm had passed, Longboat Key was declared as a disaster zone. The storm removed old docks, wharves, and boathouses from the bay. The track of t he cyclone landward of the shoreline and the counterclockwise rotation of the storm would have generated predominantly offshore winds with a strong northerly componen t. It is reasonable to believe that the slowly moving storm allowed suffi cient time to generate high waves approaching from the north. The offshore wind was likely the reason for minimal morphological evidence associated with stor m surge. This event, in conjunction with the decreasing tidal prism, is likely the reason for the substantial southward growth of the ebb tidal delta and the curve of the channel. Figure 19. Track of Hurricane Donna, 1960. 39

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In addition to the migrating shoal causing navigation problems at New Pass, Lido Key was experiencing severe erosion during the 1960s. This prompted the city of Sarasota to petit ion the US Army Corps of Engineers assistance in maintaining their inlets. The first maintenance dredge was in 1964 and subsequent dredges are listed in Table 1. The shore-perpendicular orientation of the channel, as clearly illust rated in the two example aerial photos from 1971 and 1972 (Figure 20), apparently resulted from the maintenance dredging. Figure 20. Aerial photos from 1971 and 197 2 are two examples illustrating the straight morphology and prominen t channel linear bar on the north side of the main channel. The New Pass channel has remained largely perpendicular to the shoreline since 1964 due to the well sc heduled dredging of the main channel (Figure 15). In general, the channel location and orientation have been controlled by anthropogenic activities. Ther efore, any attempt for the channel to migrate south is interrupted by the maintenance dredging. 40

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Big Sarasota Pass The Big Sarasota Pass at the present location is documented to as far back as the 1700s. The navigation map fr om 1888 positions the main channel of Big Sarasota Pass adjacent to Fishery Poin t. An inlet adjacent to Fisher Point was also documented in the 1700s. Th is suggests that the main channel has been relatively stable. Due to the long hist ory of the inlet, it is assumed to have carried much of the prism for Saraso ta Bay before the opening of New Pass (CPE, 1993a). Its domination was belie ved to have extended into the early 1900s before the massive dredge and fill construction by John Ringling (CPE, 1993a). The location and orientation of the main channel of Big Sarasota Pass has largely been stable since 1943, except for por tion just south of Lido Key (Figures 21 and 22). In 1888, the main channel was positioned approximately 200 meters to the northwest. The influence of the net southward longshor e sediment at Big Sarasota Pass is illustrated by the narro wing of the portion of the channel just south of Lido Key (Figure 22). In the 1943 aerial photo, the main channel extends through roughly the middl e of the ebb tidal delta and is relatively straight. In contrast, in the later aerial photos, t he distal channel curving to the south indicates an increased influence by the s outhward longshore s ediment transport (Figure 23). Unfortunately, there are no aerial photos availabl e from the early 1960s to illustrate the influence of Hurricane Donna. In summary, both New Pass and Big Sarasota Pass have remained at the similar positions since 1943, likely due to the stabilization of the shoreline along 41

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the south side of the inlet channels. New Pass channel orient ation is strongly influenced by the regular maintenance dredg ing, preventing the seaward portion of the channel from curving to the south. The stable channel morphology of Big Sarasota Pass, on the other hand, has not been influenced by dredging. The seaward portion of the channel is s hallower than the dredged New Pass channel. Figure 21. Example of delineated updrift edge of inlet channel at Big Sarasota Pass in 1943. A variety of events, both natural and anthropogenic, may have influence on tidal flow patterns and tidal prism. These events include the closure of Midnight Pass (approximately 10 km sout h of Big Sarasota Pass) in 1983, the maintenance dredging of New Pass, and the dredging of the Intracoastal Waterway. However, these modificati ons did not seem to have identifiable influence on the channel locations between the barrier islands. 42

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Figure 22. Big Sarasota Pass channel location and orientation from 1888 to 2005 illustrated over the 2005 aerial photos. 43

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Figure 23. Big Sarasota Pass aerial photos from 1888 to 2005. 44

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Shoreline Change Substantial erosion and accretion in the vicinity of the inlet can be observed from the rectified time-series aer ial photos. In the following discussion, a net southward sediment transport is assu med. Therefore updrift of each inlet is the north side, and downdrift is the south side. In the following sections, the delineated shoreline gathered from all the aerial photos are illustrated on a roughly decadal scale. For clarity the entire study area is illustrated as three subsections, 1) New Pass (updrift and downdr ift), 2) Big Sarasota Pass (updrift), and 3) Big Sarasota Pass (downdrift). New Pass During the 1940s little shoreline c hange occurred at the updrift of New Pass (Figure 24). Figure 24 includes shoreline change from aerial photos in 1943, 1945, 1947, and 1948. Along the updrift si de of the channel there is some erosion of the spit that extends east in to the channel. Along the southwestern tip of Longboat Key there is a small amount of accretion fr om 1945 to 1948. It is important to note t hat during the 1940s there is no offset between the updrift and downdrift shorelines around New Pass. Downdrift of New Pass, an attachm ent point appears to be inside the channel, resulting in a bulge along the downdrift side of the inlet. This protruding feature is distinguis hable into the late 1950s. Dow ndrift of the attachment along 45

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northern Lido Key, rapid shoreline erosi on is observed. There appears to be a significant flood marginal c hannel along northern Lido Key. Figure 25 illustrates shoreline change fr om 1948 to 1957. Updrift of New Pass the shoreline eroded and the spit in side the inlet remained unchanged. However, shoreline downdrift of the in let had changed substantially. The 1952 image illustrates a partially emergent set of swash bars close to the shoreline at the north end of Lido Key. The substantial shoreline adv ance in 1957 suggests that these swash bars had attached to the northern tip of Lido Key, providing a significant amount of sediment to the north end. Appar ently the attachment only benefited a limited section of t he shoreline as severe er osion just south of the attachment is measured. A groin field wa s installed along the central Lido Key in order to slow the erosion. Substantial shoreline change is observed in the 1960s, resulting from the combination of the passage of Hurri cane Donna and the first New Pass dredging (Figure 26). The 1960 aerial photo, taken two months after Hurricane Donna, illustrates a significantly different overall inlet mor phology with a large curved ebb shoal. The updrift shoreline has further eroded in the 1960 and 1961 images as compared to 1957. The 1960 and 1961 images show that the attachment point at northern Lido Key was sm oothed with some sediment transported downdrift. The smoothing of this attachment point affects only a short stretch of the shoreline, as the severe erosion around the groin field of central Lido Key persists. 46

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Figure 24. Shoreline change at New Pass from 1943 to 1948. 47

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Figure 25. Shoreline change at New Pass from 1948 to 1957. 48

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Figure 26. Shoreline change at New Pass from 1957 to 1969. 49

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Extensive changes to the New Pass system occurred between 1961 and 1969 and are best illustrated by the dramat ic shoreline change both updrift and downdrift of the inlet (Figure 26). It is reasonable to believe that the first dredging of New Pass, and nourishment of the downdrift beach in 1964, had a significant influence on the morphology. Along the updr ift side of the inlet a substantial amount of sediment had accumulated. Comparing the 1961 and 1969 aerial photos, the shoreline advanced approximately 150 m to a position similar to that of the 1940s. The spit inside the inlet acquired a significant amount of sand along the lagoon side. This is likely the re sult of artificial fill associated the accelerated developm ent of the area. The downdrift shoreline accreted substant ially at the north tip of Lido Key during the 1960s, creating a large offset between Longboat Key and Lido Key. The bulge inside the New Pass channel, as observed in the 1950s photos, has eroded along with significant erosion of 100 m at the north Lido Key headland. This dramatic morphology change is likely influenced by the 1964 dredging of New Pass. The dredging artificially creat ed a situation similar to the ebb delta breaching as described by Fitzgerald ( 1988). Following the dredge, a portion of the previously downdrift side of the ebb ti dal delta collapsed onshore resulting in the shoreline gain obse rved in the 1969 photo. The aerial photo from 1971 show s an apparent marginal channel separating the channel linea r bar from the updrift s horeline on south Longboat Key (Figure 27), between the 1971 and 1972 aerial photos. There was substantial shoreline erosion of about 70 m directly updrift of the inlet. Directly 50

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downdrift of the inlet there is significant shoreline erosion of up to 140 m between 1969 and 1971. The central Lido Key beac hes have accreted which may in part be the result of an extensive beach r enourishment project in 1970. Shoreline erosion along the northern Lido Ke y was observed between 1971 and 1972. New Pass was dredged twice in the 1970s, in 1974 and 1977, during the same year the aerial photos were taken. Comparing the 1972 and the 1974 photos, the shoreline directly downdrift and updrift of the inlet gained up to 50 m (Figure 28). The shoreline advance c ontinued to 1977 with an additional 50 m of shoreline gain. It is worth noting that the 50 m represents a maximum value of the spatially variable shoreline changes. The central Lido Key beaches show extensive accretion when compari ng the 1974 and 1977 photos. This is apparently the result of t he placement of 300,000 m3 of dredged material on Lido Key. 51

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Figure 27. Shoreline change at New Pass from 1969 to 1972. 52

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Figure 28. Shoreline change at New Pass from 1972 to 1977. 53

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Dredging of New Pass and subsequent nourishment continued into the 1980s with approxim ately 211,000 m3 of sediment placed on Lido Key. Comparing the 1977 photo with those in the 1980s, the updrift shoreline is relatively stable with gains and losses within 40 m (Figure 29). The shoreline along the inner channel remains unchanged in part due to the structuring of the south side of the channel with a cont inuous seawall. Between 1977 and 1983 the northern Lido Key headland was severely eroded, with a shoreline retreat of over 100 m, to a position where there was no longer an offset between south Longboat Key and north Lido Key. Also, following the 1977 beach nourishment, north Lido Key had experienced a shorelin e retreat of over 100 m by 1983. However, comparing the 1983 and 1986 photo s, substantial shoreline gain was observed at the headland beach. It is wo rth noting that the 1985 dredging, as visible in the 1986 photo, followed a further southward orientation (Figure 29). As typical of a post-dredging response, the swash bars over the previous downdrift ebb tidal delta are migrating and atta ching to the shoreline as is visible in the 1986 aerial photo. This resulted in substantial shoreline gain. Some alongshore spreading of the recently atta ched material can be identified in the 1986 aerial photo. 54

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Figure 29. Shoreline change at New Pass from 1977 to 1986. 55

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The 1990s saw the greatest amount of dredging in New Pass, with almost 1.12 million m3 of material removed (Table 1), in addition to an offshore dredging in 1998. Approximately 910,000 m3 of sediment were placed on Lido Key. About 300,000 m3 of sediment from the 1990 and 1993 New Pass dredging were placed onto south Longboat Key to mitigate the erosion. Substantial changes occurred between 1986 and 1990 (Figure 30). Over 50 m of shoreline erosion occurred along the updrift beach. Tremendous shoreline accretion of up to 150 m occurre d at the downdrift beach. This created an offset at New Pass. The updrift s horeline erosion continued to 1993 with another 50 m of shoreline retreat. At the downdrift beach, a portion of the accretion was eroded with a shoreline retr eat of approximately 50 m. Further downdrift the shoreline advanced. This trend continued to 1998 and is probably the result of downdrift migr ation of the attachment point, which may be related in part to the artificially controlled southw ard alignment of New Pass. The updrift shoreline advanced from 1993 to 1998, lik ely resulted from the 1993 south Longboat Key nourishment. Since 1998, shoreline change in the vicinity of New Pass has been relatively stable (Figure 31). A notabl e change occurred along the inlet channel at the updrift side between 2004 and 2005 with an up to 50 m shoreline retreat. In response, rock T-groins were placed along this shoreline to inhibit any further erosion. 56

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Figure 30. Shoreline change at New Pass from 1986 to 1998. 57

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Figure 31. Shoreline change at New Pass from 1998 to 2006. 58

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Big Sarasota Pass Due to the large offset at Big Sa rasota Pass and its immense ebb tidal delta, it is difficult to illustrate the ent ire system in one figure. Therefore, Big Sarasota Pass is separated into two parts, updrift and downdrift, for better resolution of shoreline changes. Some of the aerial photos did not cover the entire Big Sarasota Pass system. Big Sarasota Pass shoreline change also begins with aerial photos from the 1940s including 1943, 1945, and 1948. The 1943 aerial photo has a higher brightness than the rest of the aerial photo dataset, and therefore it is difficult to distinguish features. Also the lack of constructed f eatures adds to a greater uncertainty in the rectification as shown by the offset in the back bay shoreline (Figure 32). However, the overall trend in the shoreline change along the updrift side of Big Sarasota Pass can still be i dentified. Comparing the 1945 and 1948 aerial photos, the southern ti p of Lido Key had accreted with a shoreline advance of up to 110 m. The spit inside the c hannel along the updrift side also grew in size and extended further north. 59

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Figure 32. Shoreline change updrift of Big Sarasota Pass from 1943 to 1948. 60

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Downdrift of Big Sarasota Pass there is an apparent swash bar attachment at the Siesta Key headland as shown in the 1943 aerial photo (Figure 33). However the extensive beach er osion had occurred at the headland by 1948. The beaches south of the headla nd, which were already somewhat developed, had also eroded quite substantia lly, roughly 150 m to the edge of the developments. By 1948 extensive groin fields were exposed at the shoreline along north Siesta Key. Comparing 1948 and 1952 photos, the southern tip of Lido Key had accreted substantially with a shoreline gain of up to 130 m. A considerable amount of this accretion was eroded by 1957, followed by another tremendous shoreline gain of up to 160 m by 1969 (Fi gure 34). The exact reason for these large and rapid shoreline fluctuations is not clear. Coincidentally, heavy development occurred in the late 1960s resu lting in a large amount of structures on the newly accreted yet very dynamic beac h at the southern tip of Lido Key. The spit inside the channel along the updrift side had remained mostly unchanged. Much of the flood tidal delta of Big Sarasota Pass, known as Bird Key, had been filled in and developed between 1959 and 1960 (Figure 35). 61

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Figure 33. Shoreline change downdrift of Big Sarasota Pass from 1943 to 1948. 62

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Figure 34. Shoreline change updrift of Big Sarasota Pass from 1948 to 1969. 63

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Figure 35. Rapid development of Bird Key from 1959 to 1960. The channel shoreline along northern Siesta Key experienced significant erosion between the late 1940s and 1957 (Figur e 36). The beach that is visible in the 1940s photos has largely disappeared by the 1950s. At this point many of the homeowners had constructed sea walls to protect their property from the persistent erosion along the channel shor eline. In additi on to the almost continuous sea wall, some groin fields were also installed along the inlet as is visible in the 1957 aerial photo. These efforts had essentially anchored the downdrift or east side of the Big Sarasota Pass channel. Downdrift of the inlet, compar ing 1948 and 1957 (the 1952 photo did not include the downdrift beach) the headland at Siesta Ke y experienced shoreline advance of up to 60 m. This accumulati on may be the result of the deposition of sediment transported from the eroding shoreline inside the channel. This trend of accumulation had dramatically changed by 1969. The Siesta Key headland experienced severe erosi on with up to 200 m shoreline recession comparing the 64

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1957 and 1969 aerial photos (Figure 36). T he erosion can be attributed to the depletion of sediment supply due to the artificial ancho ring of the shoreline along the channel. By 1969, the entire headland is heavily stru ctured with groin fields, seawalls, and rip-rap placed along the shor eline in an effort to stop and mitigate the erosion. The ebb shoal has grown and is now bypassing and attaching in the form of swash bars further downdrift. This resulted in a shoreline gain of over 130 m at the attachment point bet ween 1957 and 1969. However, the accumulation caused by the attachment is rather local with limited alongshore spreading during this time frame. This is illustrated by the severe erosion of the shoreline updrift and downdri ft of the attachment. The large variation in shoreline position along southern Lido Key, such as that observed in the 1950s and 1960s, continues through the 1970s and 1980s. The updrift south Lido Key beach had eroded significantly with up to 170 m of shoreline retreat from 1969 to 1976 (Figure 37). Much of the development along this stretch had exposed seawall and rip-rap at the shoreline as is visible in the 1976 aerial photo. Reversing this severe erosive trend, the beach experienced accretion in 1977 and continued to 1983. This may be related to the sand supply from the 1977 and 1982 Lido Key beach nourishment. 65

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Figure 36. Shoreline change downdrift of Big Sarasota Pass from 1948 to 1969. 66

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Figure 37. Shoreline change updrift of Big Sarasota Pass from 1969 to 1983. 67

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Shoreline position at the downdri ft Siesta Key headland had remained unchanged with little to no sediment accretion during the 1970s and 1980s (Figure 38). However, over the 1 970s and 1980s a significant amount of sediment had accreted at the downdrift northern Siesta Key beach. This accretion is due to a combination of attachment point migration and some alongshore spreading of the accumula tion at the attachment point. Comparing the aerial photos from 1983 to 1990, the shoreline at the southern tip of Lido Key has advanced nearly 100 m (Figure 39). After 1990, this beach experiences an overall erosional trend with the shoreline retreating to a position similar to 1983. Figure 40 show s the shoreline change at the downdrift Big Sarasota Pass from 1983 to 1999. Ex cept for the aerial photo from 1999, all of the photos during this period of time did not extent beyond the Siesta Key headland. The shoreline at the Siesta Key headland remained relatively stable due to the structures. Compared to the aerial phot os in the 1970s, the 1999 photo illustrates that the beach ridges that were formed through the attachment of swash bars have been vegetated. This suggests that over the last 30 years this portion of the beach has been accret ionary, apparently benef iting from the sediment bypassing. 68

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Figure 38. Shoreline change downdrift of Big Sarasota Pass from 1969 to 1983. 69

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Figure 39. Shoreline change updrift of Big Sarasota Pass from 1983 to 1999. 70

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Figure 40. Shoreline change downdrift of Big Sarasota Pass from 1983 to 1999. 71

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The shoreline advance along southern Lido Key, resulting from the 2001 and 2003 beach nourishment (Table 1), is apparent when comparing the 1998 and 2004 photos (Figure 41). Subsequent beach erosion c an be identified in the 2005 and 2006 aerial photos. The southwar d longshore sediment transport is clearly illustrated by t he 2004, 2005, and 2006 aerial phot os. A portion of the eroded sand on southern Lido Key beaches apparently was deposited at the southern tip. Along the downdrift shoreli ne, the attachment of the swash bars continued in the 2000s (Figure 42). The lo w-altitude 2005 aerial photos illustrate the complicated morphology of the swash ba rs. The exact point of attachment can be related to the positi on of a particular migrating swash bar. This may be the reason for the variation in attachment locations. 72

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Figure 40. Shoreline change updrift of Big Sarasota Pass from 1999 to 2006. 73

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Figure 41. Shoreline change downdrift of Big Sarasota Pass from 1999 to 2006. 74

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Offshore Bar The offshore bar that is discussed here is that which directly interacts with the ebb tidal deltas of each inlet. The hypot hesis here is that these bars serve as important pathways for sediment bypa ssing. On many aerial photos, these offshore bars can be traced updrift to a me rging point with the shoreline, as illustrated in Figure 43. In the following discussion, this merging point is referred to as the detachment point of the offshore bar. The cr est of the offshore bar was traced to identify any trends of bar movement. Figure 43. Example of offshore bar detachment point and attachment point. The digitized offshore bar crest updrift of New Pass is illustrated in figure 44. Generally, the cross-shore distances of the bar crest increases towards the ebb tidal delta. Overall, no apparent ti me-series trend can be identified. The offshore bar updrift of Big Sarasota Pass shows a similar morphology as that 75

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updrift of New Pass (Figure 44). However, the detachment point of the offshore bar on Lido Key seems to relate to the nourishment activities. Figure 44. Offshore bar locations for south Longboat Key from visible aerial photos of New Pass. A general trend is not apparent. 76

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Figure 45. Offshore bar locations for Lido Key from visible aerial photos of New Pass and Big Sarasota Pass. A general trend is not apparent. 77

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Ebb Tidal Delta and Wave Refraction Both New Pass and Big Sarasota Pass are associated with large and active ebb tidal deltas. These two ebb tidal deltas are relatively closely spaced, but with substantially differ ent overall morphologies. The wave refraction over these ebb deltas strongly influences t he sediment bypassing patterns. As discussed above, the inlet morphodynamics is strongly influenced by southerly longshore sediment transport. In the greater study area, the net longshore sediment transport is largely controlled by the passages of cold fronts. Mehta (1996) discussed several case studies of federally maintained inlets, including New Pass. He suggested a need for extensive research on the recovery of ebb tidal deltas and the interrela tionship with the stability of adjacent beaches. A wave refraction study is emphasized as a key component of the recommended research. In the following, the steady-state s pectrum CMS-Wave model, developed by the Army Corp of Engineers (Lin et al., 2006), is used to investigate the wave refraction patterns over the ebb tidal delta. CMS-Wave is a spectral wave shallow water propagation model. Wave data collected fr om the 1994-1999 WIS dataset were examined and three representative waves were selected for use in this study. Basic wave characterist ics, including wave height, period, and direction, were input into a wave gener ation program in SMS (Surface Water 78

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Modeling System), a comprehensive modelin g interface that runs CMS-Wave. The wave generation tool in SMS creates the spectral wave input conditions used in CMS-Wave under user parameters. Bathymetry used in this model was collected in 2005 and in 2006 using RTK GP S and a precision single beam echo sounder (CPE 2005; CEC 2006). The 2-D fini te difference model, which handles wave refraction and diffraction, propagates the spectral wave over a rectilinear grid generating a 2-D visual output as well as numerical results. Results include wave height, period, direction, breaking dissipation, and radiation stresses. The characteristics of the two ebb tidal deltas are clearly illustrated by the high resolution survey (Figure 45). The Big Sarasota Pass ebb tidal delta is substantially bigger than the New Pass ebb tidal delta. In contrast to the very asymmetrical Big Sarasota Pass ebb tidal delta, the New Pass ebb tidal delta is relatively symmetrical but slightly skew ed toward the south. The main channel through New Pass is relatively short and straight, while the channel at Big Sarasota Pass is much longer and sinuous. 79

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Figure 46. Bathymetric map of study area used in modeling. Three representative wave conditions were selected based on regional meteorological and wave conditions (Fi gures 7 and 8). The main goal is to examine wave refraction patterns over the complex ebb tidal deltas under simplified conditions. Alt hough a large number of model ed runs were conducted, only the simpler cases, excluding tide influences and therefore wave-current interactions, are discussed below. The first of the three idealized wave 80

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conditions is a 1.0 m wave approaching fr om the north-northwest, representing an average post-frontal wave condition. The second wave condition is a 2.4 m wave approaching from due west, representing a very energetic storm wave condition (Figure 8). The third wave c ondition is 1.2 m wave approaching from the south, representing a relatively high energy wave condition. Figure 47 illustrates an incoming wave from a north-northwest direction, representing a typical wave condition a ccompanying a cold front passage. Wave refraction around the ebb deltas is apparent with higher waves on the northern flank of the delta and smaller waves along the southern flank of the delta due to sheltering. The wave height dissipation is closely related to the bathymetry, with less dissipation in deeper water and mo re dissipation in shallow water. 81

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Figure 47. Wave propagation of a 1.0 m wave height with a 5 s period approaching from a north-northwest direction. Figure 48 represents a relatively energet ic wave condition from a distant storm. The wave propagates toward the east from the center of the Gulf of Mexico. The wave refraction over the ebb deltas and the divergence of the wave direction just downdrift (south) of each delta are illustrated. This divergence point relates to the persistent shoreline erosi on in the north-central portion of Lido Key as discussed above. The refracted wave vectors of both ebb tidal deltas suggest the southerly trend of longshore sediment transport. Wave sheltering along the downdrift portion of the ebb deltas is eviden t. Significant wave energy reduction 82

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is modeled at both the New Pass and Big Sarasota Pass a ttachment locations. This indicates a process-response bypassing and attaching mechanism. Figure 48. Wave propagation of a 2.4 m wave height with an 8 s period approaching from due west. Figure 49 represents a prefrontal wave condition incident fr om the south. The protruding Siesta Key and the large Big Sarasota Pass ebb delta produced a large shadow zone with a smaller refr acted wave. Although Siesta Key is impacted by much of the wave energy, t he rest of the littoral system is mostly sheltered from the dissipated and refract ed waves. A divergence of the wave vectors can still be identified at the north-central area of Lido Key under a southerly approaching wave. This further explains the erosional hotspot in central Lido Key. In other words, the wave divergence occurs under both 83

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northerly and southerly approaching wave as well as shore normal waves. Also, under a southerly approaching wave, a localiz ed, relatively high wave is modeled at the headland at Siesta Key. This sugges ts that the persist ent erosion and the headland is related to the southerly approaching wave and the skewed Big Sarasota Pass channel. Figure 49. Wave propagation of a 1. 2 m wave height with a 6 s period approaching in from the south. The modeling efforts also indicated that the large variati on of the shoreline at the tip of southern Longboat Key, the southern tip of Lido Key, and Siesta Key is related to the complex wave-current interaction in addition to the wave refraction pattern discussed above. Detailed discussion of this modeling effort is beyond the scope of this thesis. 84

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Discussion The processes controlling the mor phodynamics of tidal inlets are complicated, including meteorological, tidal and wave forcing. These forces are highly variable in space and time and are difficult to quantify. In addition, they interact with each other actively. A simplified approach is to examine the morphological changes through an evaluation of a relative energy level. The following qualitative discussion on t he morphodynamics of New Pass and Big Sarasota Pass follows the above relative energy level approach. Anthropogenic alteration is a crucial factor when examining historical morphologic changes along such a modified and extensively developed coastline. Structures, dredge and fill in the bay, dredging of the channel, and nourishment activities, all have tremendous influence on the sediment bypassing system. The influence of ant hropogenic activities is clearly illustrated by morphodynamics of New Pass and Big Sarasota Pass. Both New Pass and Big Sarasota Pass ar e mixed-energy tidal inlets with New Pass illustrating a straight mor phology and Big Sarasota Pass a highly offset morphology. Also, New Pass is dredged regularly, whereas Big Sarasota Pass has never been dredged. A substantial amount of bypassing occurs at both inlets based on the analysis of historical photos. However, the pathways and 85

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mechanism of the bypassing are differ ent at New Pass as compared to Big Sarasota Pass. The bypassing at New Pass can be expl ained by the modified model 3, i.e. ebb tidal delta breaching, of Fitzger ald (1978). The ebb tidal delta breaching here is initiated by channel dredging as opposed to natural processes described in Fitzgerald (1978). Channel realignment essentially cuts through the ebb tidal delta, serving the same purpose as ebb tidal delta breaching, modifying the hydrodynamics. Typically after the channel dredging a portion of the downdrift ebb tidal delta migrates onshore and attach to the Lido Key head land, therefore, completing the bypassing. A conceptual model of the New Pass bypassing system is illustrated in Figure 50, in terms of relati ve forcing. The red arrows indicate the pathway of sediment bypassing around the inlet. Di fferent morphological features are dominated by different processes. The sediment supply along the updrift shoreline as well as along t he offshore bypassing bar is driven by wave forcing. This is especially true during the passages of cold fronts. This portion of the coast, as outlined in yellow, is mostly dominated by wave forcing. As the net longshore sediment transport reaches the ebb tidal delta, the sediment is redistributed by tidal flow. This portion of the ebb tidal delta al ong the updrift side of the inlet, as outlined in orange, is m odified by both waves and tides. Downdrift of the inlet channel, the complex swas h bars tend to migrate onshore. This portion of the ebb tidal delta is outli ned in yellow, and is dominated by wave forcing. The relative updrift and downdrift side of the ebb tidal delta is controlled 86

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by the channel dredging. In other wo rds the channel realignment allows a substantial portion of the s ediment on the ebb tidal; del ta to switch from the updrift side to the downdrift side. The sediment bypassing at Big Sarasota Pass does not follow Fitzgeralds (1978) models. Due to the shallow dist al part of the ebb channel, it seems that the sediment is transported across the entire shallow ebb del ta without major interruption of a deep channel. This particular morphology, without a deep channel in the distal part of the eb b delta, has been maintained by natural processes over at least the last 65 years. A conceptual model of sediment bypassing at Big Sarasota Pass is illustrated in Figure 51. The red arrows indicate the pathway of sediment bypassing around the inlet. The sediment supply along the updrift shoreline as well as along the offshore bypassing bar is driven by wave forcing. The frequent beach nourishments at Lido Key influence the Big Sarasota Pass system by artificially supplying a large amount of sediment to the ebb tidal delta. This anthropogenic influence cont rasts the maintenance dredging activity at New Pass. This portion of the coast, as out lined in yellow, is mostly dominated by wave forcing. As the net longshore s ediment transport reaches the ebb tidal delta, the sediment is redistributed by ti dal flow. This portion of the ebb tidal delta along the updrift side of the inlet, as outlined in orange, is dominated by both waves and tides. Downdrift of the inlet channel, the complex swash bars tend to migrate onshore. This portion of the ebb tidal delta is outlined in yellow, and is dominated by wave forcing. 87

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Figure 50. Illustration of pathways of nat ural sediment bypassing at New Pass. 88

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Figure 51. Illustration of pathways of natur al sediment bypassing at Big Sarasota Pass. 89

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The shoreline in the vicinity of both in lets fluctuates as much as 200m in a time scale of only few years. The shore line fluctuation is controlled by both wave and tide forcing in addition to artifici al supply from beach nourishment. The shoreline at the southern tip of Longboat Key at the updrift side of New Pass is relatively stable. At the downdrift side the shoreline position fluctuates dramatically depending upon the exact loca tion and timing of the attachment. This is controlled by the timing of the dredging and the timing of the collapse of the downdrift side of the ebb tidal delta. Typically when the ebb delta collapses, the downdrift shoreline advances substantially to a point where there is a notable offset between Longboat key and Lido Key. Following the onshore migration of the collapsed ebb delta, the sediment is quickly eroded restoring the straight morphology at New Pass. The results from the wave modeling indicate that there is a divergence zone in the central part of Lido Key. The divergence occurred under a variety of wave conditions. This explains the se vere erosion at central Lido Key and the classification of this beach as an erosional hotspot. Over the years a tremendous amount of sand has been placed along this stretch of the beach. Most of the nourished sand was transport ed south onto the Big Sarasota Pass ebb tidal delta. The shoreline in the vicinity of Big Sarasota Pass is substantially influenced by the frequent Lido Key beac h nourishment. Specifically, the advance and retreat of the shoreline at t he southern tip of Lido Key is directly related to nourishment activity. A cons iderable portion is transported along the 90

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shoreline and deposited at the southern tip of Lido Key This process can cause shoreline to fluctuate on t he order of 200 m. A large portion of the sediment is eventually transported onto the ebb tidal del ta. Another important pathway for the sediment to reach the ebb tidal del ta is along the offshore bar. The detachment of the offshore bar from t he beach and the attachment to the ebb tidal delta can be identified fr om some of the aerial photos. Two fairly persistent trends at the downdrift shoreline of Big Sarasota Pass are identified. The northern Siesta Key headland has experienced erosion since the 1960s after the sand supply from the inner channel of Big Sarasota Pass was terminated by the construction of sea walls. Over the last 40 years, no accumulation was observed at that point. However, downdrift of the headland, a persistent shoreline accretion was observed over the last 40 years. The pattern of shoreline advance is related to t he location and timing of the swash bar attachment. 91

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Conclusions Both New Pass and Big Sarasota Pass ar e mixed-energy tidal inlets with New Pass illustrating a straight morphology and Big Sarasota Pass a highly offset morphology. The shoreline in the vicinity of both in lets fluctuates as much as 200 m in a time scale of only few years. The shor eline fluctuation is controlled by both wave and tide forcing in addition to artifi cial supply from beach nourishment. The sediment bypassing at New Pass can be explained by a modified ebb tidal delta breaching model. The breachi ng is initiated by channel dredging. After the channel dredging a portion of t he downdrift ebb tidal delta migrates onshore, creating a notable offset between Longboat Key and Lido Key. This sediment is quickly eroded restor ing the straight morphology. At Big Sarasota Pass, the sediment is transported across the entire shallow ebb tidal delta with minor interruptions. This particular morphology has been maintained by natural processes ov er at least the last 65 years. 92

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Two fairly persistent trends at the dow ndrift shoreline of Big Sarasota Pass are identified. The northern Siesta Key headland has experienced erosion since the 1960s after the sand supply from the inner channel of Big Sarasota Pass was terminated by the construction of sea walls. Downdrift of the headland, a persistent shoreline accret ion was observed over the last 40 years, the pattern of which is related to the location and timing of the swash bar attachment. The results from the wave modeling indicate that there is a divergence zone in the central part of Lido Key. The divergence occurs under a variety of wave conditions. This explains the se vere erosion at central Lido Key and the classification of this beac h as an erosional hotspot. Anthropogenic alteration, dredge and fill ac tivity in the bay, dredging of the Intracoastal Waterway, shoreline stab ilization, inlet maintenance dredging, and beach nourishment are critical fa ctors when examining historical morphologic changes along an extens ively developed coastline. 93

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References Aubrey, D. G. and Giese, G. S., Editors, 1993. Formation and Evolution of Multiple Tidal Inlets. American Geophysi cal Union, Washington, DC. p. 246. Brooks, G. R., Doyle, L. J., Suthard, B. C., Locker, S. D. and Hine, A. C., 2003b, Facies architecture of the mixed carbonate/ siliciclastic inner continental shelf of west-central Florida: implic ations for Holocene barrier development. Mar. Geol., 200: p. 325-350. Bruun, P. and Gerritsen, F., 1960. Stability of Coastal Inlets. Amsterdam. Bruun, P., 1978, Stability of tidal in lets. Developments in geotechnical engineering. Coastal Engineering Consultants (CEC), 2006. New Pass & Big Sarasota Pass Inlet Management Study. Submitt ed to the City of Sarasota. Coastal Planning & Engineering (CPE) 2005. New Pass Inlet Management Study. Submitted to the City of Sarasota. Coastal Planning & Engi neering (CPE), 1993a. Big Sarasota Pass Inlet Management Plan. Submitted to the City of Sarasota. Coastal Planning & Engineering (CPE) 1993b. New Pass Inlet Management Plan. Submitted to the City of Sarasota. Coastal Planning & Engineeri ng (CPE), 1993c. Wave Refraction and Sediment Transport Study of New Pass and Big Sarasota Pass, Sarasota County, Florida. Submitted to the City of Sarasota. Davis, R. A., 1989. Morphodynamics of the west-central Flor ida barrier system: the delicate balance between waveand tide-domination. In Proceedings Symposium Coastal Lowlands Geology and Geotechnology, Dordrecht, 225235. Davis, R. A. and Andronaco, M., 1987. Hurricane effects and post-storm recovery, Pinellas County, Florida (1985-1986). Coastal Sediments Amer. Soc. Civil Eng., New York, p. 1025. 94

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Davis, R. A. and Barnard, P., 2000. How anthropogenic factors in the backbarrier area influence tidal inle t stability: examples from t he Gulf Coast of Florida, USA. Geological Society, London, Special Publications 75: p. 293-303. Davis, R. A. and Barnard, P., 2003. Morphodynamics of the barrier-inlet system, west-central Florida. Mar. Geol., 200: p. 77-101. Davis, R. A. and Gibeaut, J. C., 1990. His torical Morphodynamics of Inlets in Florida: Models for Coastal Zone Plann ing. Florida Sea Grant College Program, Technical Paper 55. Davis, R.A. and Hayes, M.O., 1984. What is a wave-dominated coast? Marine Geology 60: p. 313-329. Davis, R. A. and Kuhn, B. J., 1985. Or igin and Development of Anclote Key, West-peninsular Florida. Ma rine Geology, 63: p. 153-171. Davis, R. A., Beck, T. M., Wang, P., 2007. Sedime nts of New Pass and Big Sarasota Pass. Technical Report Submitt ed to Sarasota County. Department of Geology, University of S outh Florida, Tampa, FL. Davis, R. A., Wang, P., and Beck, T., 2007. Natural and Anthropogenic Influences on the Morphodynamics of Big Sarasota Pass. Proceedings of Coastal Sediments Am erican Society of Civil En gineers, New Orleans, LA, 1582-1588. Dean, R.G. and Dalrymple, R. A., 2002. Coastal Processes with Engineering Applications. United Kingdom: Cambridge University Press, p. 413-450. Dean, R. G., 1988. Sediment Interaction at Modified Coastal Inlets: Processes and Policies. Hydrodynamics and Sedim ent Dynamics of Tidal Inlets, D.G. Aubrey and L. Weishar, eds., New York. p. 412-439. De Vriend, P. T., 1996a. Mathematical modeling of meso-tidal barrier island coasts, Part I: Empirical and semi-empirical models. Advances in coastal and ocean engineering P. L.-F, Liu, ed., Vol.2, Worl d Scientific, River Edge, N.J., 115-149. De Vriend, P. T., 1996b. Mathematical modeling of meso-tidal barrier island coasts, Part II: Process-based simulation models. Advances in coastal and ocean engineering P. L.-F, Liu, ed., Vol.2, Worl d Scientific, River Edge, N.J., 115-149. 95 Elko, N. A., Holman, R. A., Gelfen baum, G., 2005. Quantifying the Rapid Evolution of a Nourishment Project with Video Imagery. Journal of Coastal Research. 21: p. 633-866.

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Elko, N.A., Wang, P., 2007. Temporal and Spatial Scales of Profile and Planform Adjustment on a Nourished Beach. Coastal Sediments American Society of Civil Engineers, New Orleans, LA. P. 378-391. Escoffier, F. F., 1940. The st ability of tidal inlets. Shore and Beach, 8: p.114-115. Escoffier, F. F., 1977. Hydr aulics and Stability of tidal Inlets. G.I.T.I Report 13, U.S. Army Corps of Engineers, Coas tal Engineering Research Center, Fort Belvoir, VA. Evans, M. W., Hine, A. C. Belknap, D. F., and Davis, R. A., 1985. Bedrock controls on barrier island development: We st-Central Florida coast. Mar. Geol., 63: p. 263-283. Fitzgerald, D.M., Hubbard, D.K., and Nummedal, D., 1978. Shoreline changes associated with tidal inlets along the South Carolina coast. In: Coastal Zone Symposium on Technical, Environment al, Socioeconomic, and Regulatory Aspects of Coastal Zone Management, ASCE, San Francisco, p. 1973-1974. Fitzgerald, D. M., 1984. Interactions bet ween the ebb-tidal delta and landward shoreline: Price Inlet, S outh Carolina. J. Sed. Petrology, 54: p. 1303-1318. Fitzgerald, D.M., 1988. Shoreline Eros ional-Depositional Processes Associated with Tidal Inlets. Hydrodynamics and Sedi ment Dynamics of Tidal Inlets, D.G. Aubrey and L. Weishar, eds., New York. p. 412-439. Fitzgerald, D.M., Zarillo, G.A., Johnston, S., 2003. Re cent Developments in the Geomorphic Investigation of Engineer ed Tidal Inlets. Coastal Engineering Journal, 45: p. 565-600. Gibeaut, J. C., and Davis, R. A., 1993. Statistical geomorphic classification of ebb-tidal deltas along the west-central Florida coast. Journal of Coastal Research, Special Iss ue, 18: p. 165-184. Harvey, J., 1982. An Assessment of B each Erosion and Outline of Management Alternatives: Longboat Key, Florida. Sarasota County, Florida. Hayes, M. O., 1975. Morphol ogy of sand accumulation in estuaries. Estuarine Research, Academic Press, New York, 2: p.3-22. Hayes, M. O., 1979. Barrier island mor phology as a function of tidal and wave regime. In: Leatherman, S.P. (eds.), Barrier Islands: From the Gulf of St. Lawrence to the Gulf of Mexico. Ne w York, Academic Press, p. 1-28. 96

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Kowalski, K. A., 1995, Morphodynamics and stratigraphy of Big Sarasota Pass and New Pass ebb-tidal deltas, Sarasota C ounty, Florida. Univ. South Florida, unpubl. M. S. Thesis. Kraus, N. C. (2000). Res ervoir Model of Ebb-Tidal Shoal Evolution and Sand Bypassing. J. Wtrwy., Port, Coastal, and Ocean Eng. 126(60): p. 305-313. Lin, L., Mase, H., Fumihi ko, Y., and Demirbilek, Z., 2006. Wave-Action Balance Equation Diffraction (WABED) Model: Tests of Wave Diffraction. Vicksburg, MS. ERDC/CHL CHETN-III-73. Mehta, A. J., Jones, C. P., Adams, W. D., 1976. Johns Pass and Blind pass, glossary of inlets report no. 4. Report Number 18, Florida Sea Grant College, Gainesville. p. 71. Metha, A. J. and Ozsoy, E., 1978. Inlet hydraulics; Flow dynamics and nearshore transport. In: Bruun, P. (ed.), Stability of tidal inlets, theory and engineering. Elsevier, Amsterdam. P. 83-161. Oertel, G. F., 1975. Ebb-ti dal deltas, inlets and backbarrier areas of the Dutch Wadden Sea. Senckenb. Marit. 24: p. 65-115. Stive, M. J. F., Wang, S. B., Capobianco, M., Puol, P., a nd Buijsman, M. C.,1998. Morphodynamics of a tidal lagoon and the adj acent coast. Proc. of Phys. Of Estuaries and Coastal Seas. Balkema, Rotte rdam, The Netherland s. P. 397-407. Tanner, W. F., 1960. Florida Coastal Classif ication. Gulf coast Association of Geological Science, 10: p. 149-202. Todd, T. W., 1968. Dynamic diversion Influence of longshore current-tidal flow interaction on chenier and barrier island plans. Journal of Sedimentary Petrology. 38: p. 734-746. Tidwell, D., 2005. Sedimentation Patterns and Hydrodynamics of a WaveDominated Tidal Inlet: Blind Pass, Florida. M.S. Thesis. University of South Florida. Twichell, D., Brooks, G. R., Gelfenbaum G., Paskevich, V., and Donahue, B., 2003, Sand ridges off Sarasota, Flori da: a complex facies boundary on a lowenergy inner shelf environment. Ma r. Geol., 200: p. 243-262. Van de Kreeke, J., 1972. A numerical model for the hydrodynamics of lagoons. Proceedings of the 13th Coastal Engineering Confer ence, Vol. 3, ASCE, New York. 93(4), 97-106. 97

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Van de Kreeke, J., 1988. Hydrodynam ics of tidal inlets. In: Hydrodynamics and Sediment Dynamics of Tidal Inlets D.G. Aubrey and L. Weishar eds., SpringerVerlag, New York. p. 1-23. Wang, P., Beck, T. M., Davis, R. A ., 2007. Current Measurements at Big Sarasota Pass and New Pass. Technical Report Submitted to Sarasota County. Department of Geology, University of South Florida, Tampa, FL. Wang, P., Beck, T. M., Davis, R. A ., 2007. Morphodynamics of Big Sarasota Pass and New Pass Elucidated From Time-series Aerial Photos. Technical Report Submitted to Sarasota County. Department of Geology, University of South Florida, Tampa, FL. Wang, P., Tidwell, D., Beck, T. M., Kraus, N. C., 2007. Sedimentation pattern in a stabilized migratory inlet, Blind pass, Florida. Coastal Sediments American Society of Civil Engineers, New Orleans, LA, 1377-1390. Wilhoit, J. C., 2004. Morphodynamics of B unces Pass, Florida. M.S. Thesis. Univeristy of South Florida. 98

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137 I-38


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ABSTRACT: Time-series aerial photographs from 1943 to 2006, including three bathymetry surveys from 1888, 1953, and 2006, are analyzed and compared. The locations of three morphological features, including that of shoreline, offshore bars, and channel orientation, are delineated over the historical aerial photos in order to examine the morphodynamics of the system. Anthropogenic alteration of the New Pass and Big Sarasota Pass system is a crucial factor in controlling the morphodynamics. Both New Pass and Big Sarasota Pass are mixed-energy tidal inlets with New Pass illustrating a straight morphology and Big Sarasota Pass a highly offset morphology. The sediment bypassing at New Pass can be explained by a modified ebb tidal delta breaching model with the breaching initiated by frequent channel dredging. The sediment bypassing at Big Sarasota Pass is different from that at New Pass, in that it is transported across the entire shallow ebb tidal delta with minor interruptions. This particular morphology, without a deep channel in the distal part of the ebb tidal delta, has been maintained by natural processes over at least the last 65 years. The shoreline in the vicinity of both inlets fluctuates as much as 200 m in a time scale of only few years. The advance and retreat of the shoreline at the southern tip of Lido Key is influenced by the frequent Lido Key beach nourishment. A large portion of the sediment is eventually transported onto the Big Sarasota Pass ebb tidal delta. The northern Siesta Key headland has experienced erosion since the 1960s. Downdrift of the headland, a persistent shoreline accretion was observed over the last 40 years, the pattern of which is related to the location and timing of the swash bar attachment.
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